1. Field of the Invention
The present invention relates to a semiconductor device such as a power device mounted on a power module or the like, and more particularly, to a semiconductor device having a junction diode structure requiring a high reverse recovery resistant.
2. Description of the Related Art
Recently, as energy saving is desired, power modules used for power converter or the like have been applied to a wide range of fields. For example, as illustrated in FIG. 9, the power module is configured to include a converter unit 100, a brake unit 200, and an inverter unit 300. The inverter unit 300 is configured so that an insulated gate bipolar transistor (IGBT) 301 and a free wheeling diode (FWD) 302 are connected to each other in reverse parallel.
In general, the FWD 302 used for the inverter unit 300 has a reverse recovery mode where a reverse blocking state is recovered from a forward conduction state. In a transient period of the reverse recovery mode, a power having a high voltage and a high current are applied to the FWD 302. If a resistant amount of the FWD with respect to the high voltage and high current, that is, a reverse recovery resistant amount is low, element breakdown easily occurs at a site where the reverse recovery current is concentrated. Therefore, in order to prevent the element breakdown, the FWD 302 uses a high reverse recovery resistant amount.
The structure of the FWD and the process of occurrence of breakdown in the reverse recovery mode will be described through the behavior of internal carriers with reference to a cross-sectional view illustrating main components of a semiconductor substrate of the FWD of the related art illustrated in FIG. 8. In a general configuration of the FWD, an n type silicon semiconductor substrate (hereinafter, abbreviated to an n type drift layer 101) is used, and a p type anode diffusion region 102 is selectively formed in the one main surface layer. An anode electrode 103 which is configured with an alloy such as Al—Si is in ohmic contact with a central surface of the p type anode diffusion region 102. An n type cathode diffusion layer 104 having a surface impurity concentration capable of allowing the layer to be in ohmic contact is formed on the rear surface of the n type drift layer 101, and a cathode electrode 105 which is configured with a laminated metal film of Ti/Ni/Au or the like is formed to be in contact with the surface of the n type cathode diffusion layer.
In addition, electric field alleviation structures such as an insulating film 109, a guard ring structure 106-1, a field plate structure 106-2, and a RESURF structure (not illustrated) are installed in a ring shape on a surface layer of the voltage-resistant region 106 surrounding the p type anode diffusion region 102 of the FWD in order to secure the resistant voltage and the resistant voltage reliability. Since main current flows in the region which is in contact with the anode electrode 103 on the central surface of the p type anode diffusion region 102 inside the voltage-resistant region 106, the region is called an active region 107. A peripheral portion 108 of the p type anode diffusion region 102 of the active region is covered with the above-described anode electrode through an insulating film such as PSG.
The load of the power module including a configuration where the FWD 302 and the IGBT 301 are arranged in reverse parallel to each other is an inductance generally representing a motor. As illustrated in FIG. 9, a return current also flows in the FWD 302 in response to ON/OFF by gate control of each IGBT 301. The initial state of the FWD 302 is a blocked state and a reverse-biased state.
When the return current flows, first, the FWD 302 having the above-described configuration is forward-biased. As illustrated in FIG. 8, in the forward-biased FWD, if the potential of holes in the p type anode diffusion region 102 exceeds the diffusion potential (internal potential) of the pn junction, the holes as minority carriers are injected from the p type anode diffusion region 102 into an n− layer (same as the n type drift layer 101). As a result, in the n type drift layer 101, the electric conductivity is changed according to the high concentration of injected hole carriers, so that the concentration of the electron carriers (majority carriers) is increased. Therefore, as seen from a well-known forward I-V curve of a diode, the forward characteristic occurs where the resistance is greatly decreased and the forward current is greatly increased.
Next, when the FWD 302 is reverse-biased, the minority carriers (holes) remaining in the n type drift layer 101 are recombined to the majority carriers (electrons) and are extracted to the anode (negative electrode) side, so that a depletion layer is spread in the n type drift layer 101. If the depletion layer is entirely spread, the FWD 302 is in the voltage blocked state. This process is called reverse recovery. The current in the above-described carrier extraction process during the reverse recovery period is called a reverse recovery current in a macro scale. In this state, the current is overflowed although the FWD 302 is reverse-biased. As a current reduction rate of the reverse recovery current is large at the time of transitioning from the forward bias to the reverse bias, the peak value of the current is increased (it is called hard recovery).
When the minority carriers (holes) are drawn out (or extracted) from the anode electrode 103 which is a negative electrode during the reverse biased period, the minority carriers are concentrated on a curvature portion 130 of the end portion of the peripheral portion 108 in the p type anode diffusion region 102. This is because equipotential lines of the electric field occurring due to the reverse bias are condensely localized in the curvature portion 130 so that the electric field density is increased, and thus, the current density and the electric field intensity are increased (particularly, when the current reduction rate is large at the time of transitioning from the forward bias to the reverse bias).
In addition, another reason why the current is concentrated on the curvature portion 130 during the reverse recovery period is that, when the main current flows in the FWD, a large amount of minority carriers exists in the lower portion of the voltage-resistant region 106 of the peripheral portion surrounding the p type anode diffusion region 102 as well as the lower portion of the p type anode diffusion region 102. The carriers of the peripheral portion are drawn into the end portion of the anode electrode 103 due to the high localized electric field during the reverse recovery period, so that the current is concentrated on the curvature portion 130.
As a method for alleviating the extraction of the minority carriers concentrated on the curvature portion 130, there is known a method having a configuration where the anode electrode 103 which is in direct contact with the surface of the p type anode diffusion region 102 is recessed in the central direction of the region by a diffusion length of the minority carriers or more. According to the configuration, the p type anode diffusion region 102 of the recessed portion is covered with the insulating film 109 to function as a resistance region (refer to ABSTRACT and FIG. 1 of JP 9-232597 A).
In addition, a document discloses that an area of a portion of an anode electrode overlapping an insulating film on a surface of a peripheral portion in a p type anode diffusion region is configured to be smaller than the insulating film area thereof, so that the reverse recovery resistant amount is improved. In addition, the document also discloses a diode where a surface width of the peripheral portion of the p type anode diffusion region is configured to be large, the insulating film is exposed without forming an anode electrode thereon or an insulating film of the exposed portion is opened, and an electrode having a floating potential is formed (refer to ABSTRACT AND FIG. 1 in JP 2010-50441 A).
FIGS. 2A and 2B are schematic diagrams illustrating a structure of a diode disclosed in JP 2010-50441 A. FIG. 2A is a plan view illustrating a portion of a plane pattern of a p type anode diffusion region; and FIG. 2B is a cross-sectional view illustrating main components of a surface portion and a surface structure of the portion corresponding to the p type anode diffusion region. An anode electrode 7 which is configured with an Al—Si alloy, an anode isolation electrode 5 (first metal film) which is configured with the same Al—Si alloy, and a field plate 6 (second metal film) are formed to have respective direct-contact portions on the surface of the p type anode diffusion region 1 through an opening portion 4 of an insulating film 3 (PSG: a phosphorus silicate glass). The anode electrode 7 and the anode isolation electrode 5 (first metal film) are electrically isolated from each other by an insulating film 3a. A p type ring-shaped region 1-1 at the outer circumferential end 1a of the p type anode diffusion region 1, which is installed in the outer-side voltage-resistant region 40, is formed in a ring-shaped plane pattern surrounding the outer circumference of the p type anode diffusion region 1 and is in contact with the field plate 6 covering the insulating film (PSG) 3 through the opening portion 4 which is installed in the insulating film (PSG) 3 covering the surface. An active region 30 where main current flows is the region where the p type anode diffusion region 1 and the anode electrode 7 are in direct conductive contact and is the central side of the insulating film 3a (right side of the figure).
In addition, there is a technique for improving an avalanche resistant amount by installing an n type non-diffusion corner region in an arch-shaped curved portion at four corners of a chip so as to be extended along the curved portion at an outer side of an anode contact region which is conductively connected to an anode electrode (refer to ABSTRACT and FIGS. 1 to 4 of JP 2011-171363 A).
Disclosed is a diode having a configuration where a convex portion which protrudes at a voltage-resistant region side, as a semiconductor substrate is seen in a plan view from a front surface side, is formed in a contact portion where an anode electrode and an anode semiconductor region are in ohmic contact with each other (refer to ABSTRACT and FIG. 1 of JP 2011-49399 A).
In addition, disclosed is a diode including a main anode region which is formed in a surface layer of an N-semiconductor layer, an isolation anode region, an anode connection region, and an anode electrode which is formed on the main anode region. The main anode region of the diode has a substantially rectangular outer circumference. The isolation anode region is formed in a ring shape along the outer circumference of the main anode region. Any one of the inner circumference of the isolation anode region and the straight line portion of the main anode region, which face each other, is configured to protrude, and the anode connection region is in point contact with the other (refer to ABSTRACT and FIGS. 1 to 4 of JP 2011-171401 A).
The diode (FWD) disclosed in JP 9-232597 A has a configuration where the insulating film 109 is installed on the surface of the peripheral portion 108 of the p type anode diffusion region 102 and the active region 107 which is in direct contact with the anode electrode 103 is recessed at the central side. In addition, the recession length of the anode electrode 103 is configured to be longer than the diffusion length of holes in the n type drift layer 101, so that the sheet resistance is increased. The current concentration at the curvature portion 130 of the peripheral portion 108 of the p type anode diffusion region 102 is suppressed by the sheet resistance.
However, as illustrated in FIG. 8, during the reverse recovery period of the diode, the occurrence of potential difference between the peripheral portion 108 of the p type anode diffusion region 102 and the p type anode diffusion region 102 at the central portion which the anode electrode 103 is in contact with may cause some problems. For example, there may be a through-hole or a thin film portion caused by some defects in the insulating film 109 in the peripheral portion of the anode electrode 103 which is in contact with the peripheral portion 108 of the p type anode diffusion region 102 through the insulating film 109. In this case, dielectric breakdown may occur, so that the p type anode diffusion region 102 and the anode electrode 103 may be short-circuited.
If the short-circuit occurs in the through-hole of the above-described insulating film 109, the amount of holes injected from the short-circuited portion is increased at the time of turn-on, the discharging of the minority carriers (holes) which are spread in the outer circumference portion (voltage-resistant region 106) within the diode is concentrated during the reverse recovery period. Therefore, the effect of alleviating the current concentration at the curvature portion 130 of the p type anode diffusion region 102 disclosed in JP 9-232597 A is not sufficient, so that the possibility of element breakdown is increased again. In many cases, the above-described defects such as the through-hole occurring in the insulating film 109 may be caused by extraneous substances, scars, or the like in a wafer process. In general, it is considered to be very difficult that the insulating film 109 having no defects is formed in the wafer process.
In addition, in the configuration of increasing the sheet resistance of the peripheral portion 108 of the p type anode diffusion region 102 disclosed in JP 9-232597 A, since the width of the peripheral portion 108 having no relation to the current capacitance of the chip is configured to be large, the chip size is increased in response to an increase in a reverse recovery resistant amount, so that the cost is increased. Therefore, a configuration capable of suppressing the reverse recovery current without increasing the width of the peripheral portion 108, if possible, is preferred.
In addition, the structure (FIG. 2) disclosed in JP 2010-50441 A is effectively used as a method of allowing no through-hole to occur on the above-described peripheral portion 108. However, since equipotential lines during the reverse recovery period are changed according to a shape of the anode isolation electrode 5, the influence on the reverse recovery resistant amount is considered.